1. Field of the Invention
The present invention is directed to a method for printing with a multi-level character generator. The invention is also directed to a printer device that is explained in greater detail below.
2. Description of the Related Art
A method is known wherein an optical character generator illuminates a photoconductor with at least one light source. Light encoding data that respectively contain one of at least three different light encoding values are generated from print data of a print image. The light encoding values are respectively allocated to a different illumination energy value with which the character generator illuminates the photoconductor.
In contrast to traditional bi-level character generators, character generators driven with more than two light encoding values are referred to as multi-level character generators. Although there are more than two light encoding values given multi-level character generators, there are ultimately only printed or non-printed surfaces. Compared to bi-level character generators, however, multi-level character generators offer the possibility of designationally defining the size of the charge regions and, thus, of the pixels (picture elements) in order to produce the impression of different gray scale values for someone who views the developed charge image. Such a multi-level character generator is disclosed by U.S. Pat. No. 5,767,888.
What is disadvantageous about the known printing with multi-level character generators is that the quality is reduced given changing printing conditions. For example, these printing conditions include the increasing age of the photoconductor and changes in the quality of the toner or, respectively, developer. Methods with which print images having good quality can be printed even given modified printing conditions are employed for printers with bi-level character generators, see, for example, the balancing method set forth in Published International Patent Application WO 97/37285.
Given LED character generators, moreover, auxiliary parameters with whose assistance manufacture-conditioned deviations in the light emission of the individual LEDs are compensated is usually prescribed for each LED. All LEDs therefore emit the same illumination energy given the same light encoding value. Such a balancing is disclosed, for example, in European Patent EP 0 275 254 B1.
German Published Application DE 37 27 808 A1 discloses an image recording device wherein topical deviations of the sensitivity of a photoconductor are determined by comparison to a reference potential and are subsequently corrected. The correction ensues topically dependent with different correction quantities when printing various regions of the print image.
U.S. Pat. No. 4,709,250 discloses a method for printing with the assistance of a character generator. The potential V is acquired at a given balance illumination energy, whereby the balance illumination energy derives from the current intensity flowing through the laser or LED together with the illumination time on the photoconductor. The acquired potential V is compared to a reference potential. The illumination energy values are modified on the basis of this comparison.
An object of the present invention is to provide a method for the operation of an electrophotographic printer or copier device with multi-level character generation that makes it possible to generate print images having high print quality even given changing printing conditions. Further, a printer device with which the method can be implemented is provided.
The objects of the invention are achieved by a method for printing with a character generator,whereby an optical character generator illuminates a photoconductor with at least one light source, light encoding data are generated from print data of a print image, the light encoding data respectively contain one of at least three different light encoding values that are allocated to different illumination energy values, a balance potential to be set on the photoconductor is prescribed for a balance event, the illumination energy producing the prescribed balance potential on the photoconductor is acquired as balance illumination energy, and whereby the illumination energy values to be generated by the character generator given the various light encoding values are modified in the same relationship dependent on the acquired balance illumination energy, whereby the degree of the modification is determined by the deviation of the balance illumination energy from a reference illumination energy that generates the balance potential given employment of a reference photoconductor with a predetermined discharge characteristic.
In a preferred embodiment, a correction factor is determined that is a criterion for the deviation of the current characteristic of the photoconductor from the predetermined discharge characteristic; and the illumination energy values are modified with the assistance of the correction factor. Specifically, the amount of the balance potential or, respectively, of the acquired balance potential deviates by less than 100 V from the amount of the discharge potential of the photoconductor. The amount of the predetermined balance potential or, respectively, of the acquired balance potential of a preferred embodiment lies no more than 50 Volts from the average of the amount of the discharge potential of the photoconductor and the amount of the discharge potential. The correction factor for setting the character generator is determined preferably by division from the acquired balance illumination energy and the reference illumination energy or, respectively, from the acquired balance potential and the reference potential. The acquired balance illumination energy or, respectively, the acquired balance potential may be employed as a correction factor for the setting of the character generator. A current for the drive of the light sources or, respectively, of the light sources of the character generator or their on duration is modified dependent on the correction factor. An approximation for the characteristic of the photoconductor is preferably employed for determining the deviation or for determining the correction factor. The following approximation is employed:
VD(K,T,H)=(VC-VLIM)xc2x7exp(xe2x88x92Kxc2x7Txc2x7H)+VLIM,xe2x80x83xe2x80x83(1)
whereby
VC is the charge potential of the photoconductor in volts,
VD is the discharge potential of the photoconductor in volts,
VLIM is the lowest obtainable discharge potential in volts,
H is the illumination energy in xcexcWs/cm2,
T is the currently acquired temperature of the photoconductor in xc2x0 C.,
K is the photoconductor class in cm2/(xcexcWsxc2x0 C.), and
exp is the exponential function.
In one aspect of the invention, the balancing event is automatically implemented, preferably after a printer or, respectively, copier device is turned on and/or after longer printing pauses and/or after longer printer operation and/or on demand of an operator. A further feature provides that the discharge characteristic of the reference photoconductor is permanently prescribed, particularly independently of modifications due to aging or modified ambient conditions in the printing process.
The present invention also provides a printer or copier device, particularly for the implementation of the preceding methods, including a print data unit that generates light encoding data having respectively one of at least three different light encoding values from print data of a print image, a multi-level character generator that is driven with the light encoding data and emits a predetermined illumination energy dependent on the respective light encoding value, and a photoconductor that is discharged by the illumination energy output by the character generator, whereby a balancing device is provided that, in an automatic balance event, modifies the illumination energies emitted by the character generator at different light encoding values in the same relationship, means are provided for the balance event in order to prescribe a balance potential to be set on the photoconductor, means are provided that acquire the illumination energy producing the predetermined balance potential on the photoconductor as balance illumination energy, the illumination energy values to be generated by the same generator given the various light encoding values are modified by the balance device in the same relationship dependent on the acquired balance illumination energy, and whereby the degree of the modification is determined by the deviation of the balance illumination energy from a reference illumination energy that generates the balance potential given employment of a reference photoconductor with a predetermined discharge characteristic.
The invention is based on the perception that, due to the incomplete linear characteristic of the photoconductor, the illumination energy is actually to be individually balanced for each light encoding value in the balancing of a multi-level character generator. However, there is also an adequately exact balancing for many applications when a common balancing procedure is implemented for the illumination energies of the different light encoding values. In the inventive method, the illumination energies generated given the different light encoding values are therefore modified in the same relationship. Most of the light encoding values have illumination energies that lie on a linear region of the photoconductor characteristic, so that the illumination energies for the different light encoding values can be set with adequate precision as a result of the correction ensuing in the same relationship.
Given the method of the invention, a balancing potential to be set on the photoconductor is prescribed for the balancing event. Subsequently, the illumination energy that produces the predetermined balancing potential on the photoconductor is acquired and a balanced illumination energy determined. Dependent on the acquired balance illumination energy, the illumination energy values to be generated by the character generator given the various light encoding values are modified in the same relationship.
The extent of the modification is determined by the deviation of the balance illumination energy from a reference illumination energy. The reference illumination energy is the illumination energy that generates the balance potential given employment of a reference photoconductor with a predetermined discharge characteristic. The discharge characteristic indicates the relationship of illumination and potential. The reference photoconductor, for example, is a photoconductor in a new printer device that is operated at 20xc2x0 C. room temperature.
In the inventive method, the illumination energy values generated by the character generator at the different light encoding values are modified in the same relationship relative to one another and are thus reset. The inventive method is simple because all illumination energies are modified in the same relationship. Nonetheless, print images having a print quality that is sufficient for many purposes derive. The higher print quality that can be achieved given a multi-level character generator compared to a bi-level character generator is also assured unmodified by the inventive method when the properties of the photoconductor and/or of the electrographic development system change.
In one development, a correction factor is determined that is a criterion for the deviation of the current characteristic of the photoconductor from the predetermined discharge characteristic. The illumination energies are then modified in a simple way with the assistance of the correction factor. This ensues in that, for example, the illumination energy value predetermined for each light encoding value is multiplied by the correction factor.
In another development of the inventive method, the amount of the predetermined or, respectively, acquired balance potential deviates by less than 100 V from the amount of the discharge potential of the photoconductor. For example, the amount of the balance potential lies only slightly above what is the lowest discharge potential in terms of amount. In other words, the discharge potential is the potential that is established on the photoconductor when the photoconductor is completely discharged. Given certain photoconductors, the discharge potential is relatively independent of the printing conditions. For example, it hardly changes with increasing age of the photoconductor or with different temperature. The discharge potential is therefore especially well-suited for identifying the balance illumination energy.
In an alternative development of the inventive method, the amount of the predetermined balance potential lies roughly at the average from the amount of the charge potential of the photoconductor and the amount of the discharge potential and thus lies in the linear region of the photoconductor characteristic. In particular, only this linear part of the photoconductor characteristic is employed in the printing given high-speed printing.
In a next development, the correction factor is determined by division of the acquired balance illumination energy by the reference illumination energy. Alternatively, however, the acquired balance illumination energy or, respectively, the acquired balance potential can also be immediately used as a correction factor for the character generator.
In one development, an approximation for the characteristic of the photoconductor is employed for the determination of the illumination energies to be emitted by the character generator. The involvement of the characteristic makes it possible to implement only a single measurement at the photoconductor. For a predetermined illumination energy, for example, the potential occurring given an illumination of the photoconductor with this illumination energy is measured. Subsequently, the balance illumination energy belonging to the predetermined balance potential or, respectively, the balance potential belonging to the predetermined balance illumination energy is determined from the characteristic, which changes with the printing conditions.
The balancing event is automatically implemented. This preferably ensues after a printer or, respectively, copier device is turned on, after longer printing pauses, after a longer printing operation and/or on demand of an operator. These measures assure that modifications of the photoconductor due to aging or due to altered ambient conditions are taken into consideration in the printing process.
In one development, the illumination energy values are modified in the same relationship both with respect to one another as well as for picture elements. The correction thus identically influences all picture elements.
The invention is also directed to a printer or, respectively, copier device that, in particular, is employed for the implementation of the inventive methods or, respectively, their developments. The aforementioned technical effects thus also apply to the inventive printer or, respectively, copied device.